Impulse drying occurs when a wet paper web, carried on a water absorbing felt, passes through the press nip of a pair of rolls, or a roll and shoe, in which a roll is heated to a high temperature. Impulse drying may also be accomplished using a linear press with flat platens, in this case one platen is heated and the other may be at ambient temperature. It is projected that wide commercialization of impulse drying would result in a large industry wide energy savings.
In addition to the impact on energy consumption, impulse drying also has an impact on paper sheet structure and properties. Surface fiber conformability and interfiber bonding are enhanced by transient contact with the hot pressing surface. Impulse drying produces a distinctive density profile through the sheet that is characterized by a dense outer layer. This translates into improved physical properties for many grades of paper. The persistent problem with the use of impulse drying is that as the press load is released, the pressure exerted on the heated fluid inside the web is reduced and flash evaporation can occur inside the web. The result is that the web delaminates. This is particularly a problem with heavy weight grades of paper. It has been a major constraint in the commercialization of impulse drying.
It has been reported, Crouse, et al. "Delamination: A Stumbling Block to Implementation of Impulse Drying Technology for Liner Board", TAPPI Engineering Conference, Atlanta, Ga. September 1989, that various degrees of delamination were experienced with liner board dried at press roll surface temperatures above 150.degree. C. (300.degree. F.). When delamination was avoided by operating at temperatures below 150.degree. C. (300.degree. F.), water removal efficiencies were not significantly different than those obtained by conventional pressing. It was concluded in this paper that to realize the potential of impulse drying it would be necessary to alleviate delamination.
In laboratory scale simulations, Laverly, H. P., "High Intensity Drying Process - Impulse Drying Report Three" DOE/CE/40738-T3, February 1988, it was found that increased pulp refining encouraged delamination and it was postulated that thick or highly refined sheets exhibit greater resistance to the flow of vapor than thin or unrefined paper webs. Thick and refined paper webs have a high specific surface and therefore a high flow resistance. When the press load is released, high vapor pressures are produced internal to the web because the vapor cannot readily escape the web. If the pressure is high enough, the web structure fails and the web delaminates. Reducing the temperature of the press surface eliminates delamination, but also reduces water removal to the point that the impulse drying process is no more efficient than standard double felted pressing.
Orloff, D. I., in "Impulse Drying Control of Delamination" and U.S. Pat. No. 5,101,574 shows that reducing the thermal diffusivity of the heated press surface reduces the probability that delamination will occur. Thermal diffusivity is the K/.rho.C.sub.v where K is the thermal conductivity, .rho. is the density and C.sub.v is the specific heat. The magnitude of this quantity determines the rate at which a body with a nonuniform temperature approaches equilibrium. The units of thermal diffusivity, after cancelling like terms are meter.sup.2 per second (m.sup.2 /s).
It is explicitly stated by Orloff that the press surface must be impermeable to steam. If a porous material is used to reduce the thermal diffusivity of the press surface, the characteristic density profile of impulse drying is not produced. Orloff shows that a non-permeable, low thermal diffusivity press surface allows higher press surface temperatures to be used for some furnishes, as compared to a high thermal diffusivity surface. A typical high thermal diffusivity surface is steel. A low thermal diffusivity surface can be produced using ceramics, polymers, inorganic plastics, composite materials and cermets. At the higher press surface temperatures made possible by a low thermal diffusivity surface, the water removal efficiency of impulse drying exceeds that of double felted pressing. A low thermal diffusivity press surface will produce web delamination if the heated press surface is at too high a temperature.
It is the principal object of the present invention to provide a method and apparatus for heated surface pressing and impulse drying which inhibits web delamination at heated press surface temperatures ranging from the ambient boiling temperature of the internal web liquid to temperatures in excess of the critical point temperature of the internal web liquid. The method and apparatus are effective at inhibiting web delamination regardless of press surface thermal diffusivity, web internal structure, web basis weight, or web internal liquid.